Corrosion-resistant double-coated steel material

ABSTRACT

Zinc-coated steel materials in general use which have undergone the chromate treatment for improved corrosion resistance suffer from a disadvantage that the corrosion-resistant coating is susceptible to flexure and cracks or peels off at flexures, with the result that the protective function of chromate is lost. The improved corrosion-resistant double-coated steel material to which the present invention relates is obtained by coating in a fusion-bonded state tin or tin-based alloy onto a zinc coating applied to the surface of steel material or by applying a zinc coating on the surface of steel material and further applying in a fusion-bonded state a coating of tin or a tin-based alloy. Thus obtained double-coating free from defects which chromate has, increases in mechanical strength and improves corrosion resistance to an unexpected extent.

1451 Dec. 31, 1974 CORROSION-RESISTANT DOUBLE-COATED STEEL MATERIAL [75]Inventor: Naomi Kubu, Nagoya, Japan [73] Assignee: Usui Kokusai SangyoKabushiki Kaisha, Sumo-gun, Shizuoka Prefecture, Japan 221 Filed: Dec.22, 1971 21 Appl. 190.; 211,087

[30] Foreign Application Priority Data May 10, 1971 .lapan.... 46-30959[52] U.S. C1. 29/1965 [51] Int. Cl 823p 3/10, B32b 15/18 [58] Field ofSearch. 29/1965 2 [56] References Cited UNITED STATES PATENTS 933,6129/1909 Allen 29/1965 2,057,762 10/1936 Boone et 31.... 29/1965 2,258,32710/1941 Kramer...., 29/1965 2,428,033 9/1947 Nachtman 204/37 3,231,1271/1966 Virzi 29/1965 3,323,938 6/1967 Vaught 29/1965 Primary Examiner-W.Stallard Attorney, Agent, or Firm-Irving M. Weiner 5 7] ABSTRACTZinc-coated steel materials in general use which have undergone thechromate treatment for improved corrosion resistance sufier from adisadvantage that the corrosion-resistant coating is susceptible toflexure and cracks or peels off at flexures, with the result that theprotective function of chromate is lost.

The improved corrosion-resistant double-coated steel material to whichthe present invention relates is obtained by coating in a fusion-bondedstate tin or tin-based alloy onto a zinc coating applied to the surfaceof steel material or by applying a zinc coating on the surface of steelmaterial and further applying in a fusion-bonded state a coating of tinor a tin-based alloy. Thus obtained double-coating free from defectswhich chromate has, increases in mechanical strength and improvescorrosion resistance to an unexpected extent.

3 Claims, No Drawings CORROSION-RESISTANT DOUBLE-COATED STEEL MATERIALThis invention relates to a novel corrosion-resistant coated steelmaterial obtained by improving corrosionresistant coatings such as zincand tin or tin-based alloys applied onto steel materials such as pipe,rod, plate, and wire. v

Zinc-coated steel, materials with chromate treatment have hithertobeenvin general use for their outstanding corrosion resistance, but thiscorrosion-resistant coating is susceptible to flexure and cracks orpeels off at flexures during bending,.with the result that theprotective function of chromate is lost.

This invention provides a coated steel material which is endurable tobending and exhibits the extremely outstanding corrosion resistancewithout chromate treatment. This invention is more particularlyconcerned with corrosion-resistant double coated steel materialsprepared by applying a zinc coating and subsequently applying in afusion-bonded state a coating of tin or a tin-based alloy (simply calledan outer coating hereunder) onto said zinc coating. This invention hasbeen made based on our finding that the above-mentioned double-coatingwhich has never occurred to anybody in the past exhibits the outstandingcorrosion-resistance.

In embodiments of this invention the zinc coating means of melt-plating,electro-plating, or metalizing.

The zinc coating is corroded by the liquid metal corrosion (abbreviatedto L'MC hereunder) when tin or a tin based alloy is double-coatedthereon by fusion bonding, but it is possible to allow the 'zinccoatingto stay as the inner layer between the surface of steel materiala tin-based alloy onto the zinc coating provided on the surface of steelmaterial.

3. When the double-coated steel material is subjected to the salt-waterspray test, white spots appear before it gets rusty.

The corrosion resistance of the corrosion-resistant double-coated steelmaterial with which this invention is concerned is improved by providingthe abovementioned outer coating, in a fusion-bonded state, onto theabove-mentioned inner coating. The outer coating is formed usually byallowing the steel material provided with the inner coating to soak inor pass through a bath for the outer coating. Where the uniformity ofcoating thickness is required,'the inner and outer coatings are formedin two layers by means of electroplating and subsequently heat-treatingthus obtained electro-plated double coatings. In this case the outercoating of tin-plating is chosen for the inner coating of Samples ofdouble-coated steel pipe prepared according to the invention and samplesof single-coated steel pipe prepared by coating zinc, tin, or atin-based alloy alone, were subjected to the salt-water spray test. Thetest results shown in the following tables indicate the outstandingcorrosion resistance of the doublecoated steel pipe prepared accordingto the invention.

Ex. 1, 2, 3, 4, and 5 in the tables correspond respectively totheexamples described below and denote respective samples obtained inthe examples. Also, Comp. 1, 2, 3, 4, and 5 in the tables denote samplesof single-coated steel pipes subjected to the salt-water test. Thesecomparison samples were coated by an orand the outer coating by loweringthe temperature of dinary method, which is omitted in thisspecification.

Table 1 Results of Salt-Water Spray Test Sample Coating Time Ex. 1Doublelnner Fusion bonded Zn coated Coating thickness 2 Outer Fusionbonded Sn not lW 6W 7B 10R Coating thickness 6 observed Total 8p. Comp.l Same as not 3W SW 158 15R observed Ex. l

tin or tin-based alloy bath, increasing the speed of steel material topass through the bath, or shortening the immersion time.

The corrosion resistance achieved by the corrosionresistantdouble-coated steel material with which this invention is concerned issurprisingly outstanding and can not be expected from the corrosionresistance of steel materials simply coated with zinc, tin, or atinbased alloy. It is considered from the following facts that suchcorrosion resistance is obtained because pin holes on the outer coatingare clogged by zinc carbonate and zinc hydroxide.

l. Coating of zinc, tin, or a tin-based alloy alone on the surface ofsteel material does not impart such supe- The test pieces made in theexample mentioned rior corrosion-resistance asthat achieved by thisinven- 6 5 below were tightly plugged with a synthetic resin piecestion. v 2. Unexpected outstanding corrosion resistance is' obtained bydouble-coating, by fusion bonding, tin or at each end, were defattedon-the surface and were hung at an angle of inclination of 30 degrees atintervals of 10 mm. in a sealed chamber which was filled with thebelow-mentioned salt water as-a spray. The

variation with the lapse of time of the rust produced on.

the surface of each test'pieces in the atmosphere of this' salt waterspray was observed and reconded.

Summary of the used salt water and apparatus:

(a) Purity: Refined sodium chloride of purity of more than 99.5%.

(b) Salt water prepared by dissolving of the refined sodium chloride indistilled water was used at 35C. The salt water at 35C was of a specificgravity of 1.020 and pH of 6.9.

(c) The sealed chamber was of a capacity of 0.34 m. The amount of thesprayed salt water was 3.8 liters/24 hrs.

RR: Streaky red rust spots 5 SO on.

For instance, All W indicates a state in which the entire surface ofcoating is covered by fine white matters, and All [/2 RR indicates astate in which a half of the surface is' covered by streaky red rustspots. These matters appear in the order Table 2 Results of Salt-WaterSpray Test Sample Coating Time 48 hr I68 336 672 l 176 Ex. 2 DoublelnnerFusion bonded Zn coated Coating thickness 7p. not not Outer Fusionbonded Sn-Pb* obob- 2W 5W 9R Coating thickness 10p. served served Total17p. Comp. 2 Same as lW 5W 12W 17B 17R The componentratio o l Sn-Pb is80:20.

(d) The spraying device consisted of two nozzles. (Note 1) The immersiontime of alloy bath in Ex.2 is 0.5 seconds and the immersion time ofalloy bath in The spraying pressure was 1 kg./cm 5) The tests wererecorded by the following marks.

Comp.2 is 1.5 seconds.

Table 3 Results of Salt-Water Spray Test Sample Coating Time 48 hr 168336 672 H76 Ex. 3 Double- Inner Electro-plated Zn coated Coatingthickness 2p. not not Outer Fusion bonded Sn-Pb* obob- SW 78 lSR Coatingthickness 10p. served served Total 12p. Comp. 3 Same as 4W llW 5B 3R 25RThe component ratio of Sn-Ph is 80:

(Note) 1) The immersion'time of alloy bath in Ex.3 is

50 Comp.3 is 2 seconds.

Table 4 Results of Salt-Water Spray Test 0.5 seconds and the immersiontime of alloy bath in Sample Coating Time 48hr I68 336 672 H76 Ex. 4Doublelnner Fusion bonded Zn coated Coating thickness 6 not Outer Fusionbonded Sn-Cd* ob- 2w 6W 5B 4R Coating thickness 7 served Total 13 Comp.4 Same as not 2W 7W 78 SR Comp. 3 ob served Comp. 5 Single- Fusionbonded Sn-Pb All W 5R 6RR l7RR coated Coating thickness 8p.

' The component ratio of Sn-Cd is :30.

Table 5 Size is the same as that in Example I. b. Molten flux Same asthat used in Example 1.

Results of Salt-Water Spray Test Sample Coating Time Ex. 5 DoublelnnerElectro-plated Zn coated Coating thickness 13p.

Outer Elcctroplated Sn not not obob- 2W 48 OR Coating thickness 8p.served served Total Fusion bonded Double Thickness 19p. Comp. 5 Same asnot 1W 4W 8B 7R ob- Ex. 5 served (Note) 1) The time for passing throughthe electric 20 c. Plating bath furnace in Ex.5 is 0.25 seconds and thetime for passing through the electric furnace in Comp.5 is 1.3 seconds.

EXAMPLE 1 Raw materials:

a. Mild steel pipe 6.35 mm O.D. X 0.71 mm thick X 500 mm long X 5 piecesb. Molten flux ZnCl; 70 wt%, Nl-I Cl wt%, Temperature 350C c. Moltenzinc bath; bath temperature 470C d. Molten tin bath; bath temperature280C Preparation of double-coated steel pipe:

Five steel pipes with clean surface treated in a usual manner wereimmersed in the molten flux at 350C for 15 seconds and then immediatelyimmersed in the molten zinc bath at 470C for 10 seconds to obtain thezinc coating with athickness of about 12p. These coated steel pipes werethen immersed in the molten tin bath at 280C for about 0.8 seconds toobtain double-coated mild steel pipes with about So thick coating. Fromamong these five pipes were selected three samples with uniform coating,one for the inspection of coating thickness and two for the corrosionresistance test, one bent to Z shape and another unbent.

The inspection for coating thickness revealed that the inner coating ofzinc is about 2p thick and the outer coating of Sn-Zn alloy is about 6p.It is considered that the thickness of the inner coating was decreasedto only 2p. because the zinc coating was partly lost due to the LMCaction when the outer coating was fusionbonded.

The corrosion resistance test by the salt-water spray test gave nodifference in the occurrence of rust between the two samples tested. Inthe case of chromate pipes, craking and peeling occur at flexures, butin the case of these samples such trouble did not occur. The test resultin the column Ex. 1 in Table l is an average value of two samples. It isseen that the corrosion resistance of this example is unexpectedlyoutstanding as compared with the sample of Comp. 1 in Table l.

EXAMPLE 2 SnPb alloy with an :20 composition by weight.

Bath temperature: 240C Preparation of double-coated steel pipe:

Five zinc-coated mild steel pipes were immersed in the molten flux bathat 350C for 15 seconds and then immediately immersed in theabove-mentioned plating bath at 240C for 0.5 seconds to obtain thecoating with an average thickness of 17p. From among these five pipeswere selected three samples with uniform coating, one for the inspectionof coating thickness and two for the corrosion resistance test, one bentto Z shape and another unbent.

The inspection for coating thickness revealed that the inner coating ofzinc is about'7p. thick and the outer coating fusion-bonded thereon isabout 10p. Unlike Example 1, thinning of the inner zinc coating is notobserved in this example. This was achieved by selecting a propercomposition of an alloy for the outer coating, which allows one to lowerthe bath temperature, and by shortening the immersion time so that thezinc coating is not affected by the LMC.

The corrosion resistance test by the salt-water spray test was carriedout for two sample pipes in the same manner as in Example 1. Results areshown in the column of Ex. 2 in Table 2. As in the case of Example 1,there was no tendency that rust occurs particularly at flexures.

It is seen that the corrosion resistance achieved in this example isunexpectedly outstanding as compared with that of the sample of Comp. 2in Table 2.

EXAMPLE 3 Five double-coated steel pipes were prepared in the samemanner as in Example 2, except that electroplated zinc-coated steelpipes with a coating thickness of 3p. were used instead of fusion-bondedzinc-coated steel pipes. As in the case of Example 2, two pipes weresubjected to the corrosion resistance test and one pipe was subjected tothe inspection of coating thickness. [t was found that the inner zinccoating is about ZIL thick and the outer Sn-Pb (80:20) coating is about10p. thick, the total coating being about 12p thick.

The corrosion resistance test by the salt-water spray test was carriedout with regard to two sample pipes in the same manner as in Example 1.Results are shown in the column of Ex.3 in Table 3. As in the case ofExample l, there was no tendency that rust occurs particularly atflexures.

It is seen that the corrosion-resistance achieved in this example isunexpectedly outstanding as compared with that of the sample of Comp. 3in Table 3.

EXAMPLE 4 Raw materials:

a. Fusion-bonded zinc-coated mild steel pipes, 5

pieces Coating thickness: about 6 1.

Size is the same as that in Example 1.

b. Molten flux Same as that used in Example 1.

c. Plating bath 7 Sn-Cd alloy with a 70:30 composition by weight Bathtemperature: 220C Preparation of double-coated steel pipe:

Five zinc-coated mild steel pipes were immersed in the molten flux bathat 350C for seconds and then immersed in the above-mentioned platingbath at 220C for 1 second. As in the case of Example 1, two pipes weresubjected to the corrosion resistance test and one pipe was subjected tothe inspection of coating thickness. It was found that the inner zinccoating is about 6p. thick and the outer Sn-Cd coating is about 7p.thick, the total coating being about 13 thick.

The corrosion resistance test by the salt-water spray test was carriedout with regard to two sample pipes in the same manner as in Example 1.Results are shown in the column of Ex.4 in Table 4. As in the case ofExample I, there was no tendency that rust occurs particularly atflexures.

It is seen that the corrosion resistance achieved in this example isunexpectedly outstanding as compared with that of the sample of Comp. 4in Table 4.

EXAMPLE 5 a. Steel material tested:

Mild steel pipes; 1 piece 10 mm O.D. X 0.71 mm thick X10 m long b.Preparation of double-coated steel pipe;

The sample pipe was electro-plated with zinc to a coating thickness of13p. by the usual method. This pipe was further electro-plated with tinto a coating thickness of 8p. Thus, a double-electro-coated pipe with acoating thickness of 21g was obtained. This pipe was then allowed topass through an electric furnace having an atmosphere of ammoniadecomposition gas, in order to obtain a doublecoated steel pipe having azinc inner coating and a tin outer coating fusion-bonded to the innercoating. The average coating thickness was 19;/.. Which is about 2p.thiner than the average coating thickness before the heat treatmentcarried out at 245C and at a pass speed of 0.25 m/sec.

Then, this double-coated steel pipe was cut to 20 samples 500 mm long.Two samples were selected at random for the corrosion resistance test;one was bent to Z shape and another was not bent.

The corrosion resistance test by the salt-water spray test was carriedout for the two samples in the same manner as in Example 1. Results areshown in the column of Ex. 5 in Table 5. It is seen that the corrosionresistance achieved in this example is unexpectedly outstanding ascompared with that of the sample of Comp. 5.

What is claimed is:

1. In steel materials having corrosion-resistant double-coatingconsisting of inner and outer two layers, an improvedcorrosion-resistant double-coated steel material characterized in thatsaid inner layer is a zinc coating and said outer layer is atin-predominant coating formed in a fusion-bonded state on said innerlayer.

2. An improved corrosion-resistant double-coated steel material inaccordance with claim 1 in which the tin-predominant outer coatingconsists of tin alone.

3. An improved corrosion-resistant double-coated steel material inaccordance with claim 1 in which the tin-predominant outer coatingconsists of a tin-based alloy.

UIi'ttiD s'lATEs PA.'IEI-"I OFFICE 1 CERTIFICATE OF CORRECTION PATENTNO.3,857,684

DATED December 31, 1974 INVENTOMX) Naomi KUBU It is certttiect that arm:appears in the absve-tdentitted patent and that smd Letters Patent arehereby lortected as snot-m below: Title ag correct the inventor 5 nameto read --I ubo- (both occurrences) Title page, in the Abstract, line14, change "Thus" to -The thus--7 before "free" insert -is--; line 15,insert a comma after "strength" Column 2, line 16 insert a comma after"electroplating": before "thus" insert --the--: line 65, change "pieces"to --piece-7 line 66 insert a comma after "surface" 7 line 68, change"scaled" to sealed--. Column 3, line 3, change "pieces" to ---piece;line 4, correct the spelling of --recorded-7 line ll, after "and" insert--a---.' Column 4, line 4, change "rence" to --rences-7 line 11, change"the only" to only the--. Column 5, line 42, after "pipes" insert---there-; line 56 correct the spelling of -cracking-; line 58, change"column" to --row of--: line 60 change "unexpectedly" to -so-; line 61,change "compared with" to -can never be expected from the result of thecorrosion resistance test on-. Column 6, line 29, after "pipes" insert--there--7 lines 45-46, change "column" to --row--; line 67 change"column" to -row--. Column 7, line 30, change "column" to --row--.Column 8, lines 22-23, change "column" to -row.

Table 1, after "outer" change "Fusion bonded Sn" to --Fusion bondedSn-Zn--.

Table 4, After "Comp. 4" change "Same as Comp. 3" to -Same as EX. 4--7delete the following last two lines:

"Comp.5 Single- Fusion bonded Sn-Pb* All W 5R 6RR l7RR coated Coathingthickness 8 Signed and sealed this 10th day of June 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officerand Trademarks

1. IN STEEL MATERIALS HAVING CORROSION-RESISTANT DOUBLE COATINGCONSISTING OF INNER AND OUTER TWO LAYERS, AN IMPROVEDCORROSION-RESISTANT DOUBLE-COATED STEEL MATERIAL CHARACTERIZED IN THATSAID INNER LAYER IS A ZINC COATING AND SAID OUTER LAYER IS ATIN-PREDOMINANT COATING FORMED IN A FUSION-BONDED STATE ON SAID INNERLAYER.
 2. An improved corrosion-resistant double-coated steel materialin accordance with claim 1 in which the tin-predominant outer coatingconsists of tin alone.
 3. An improved corrosion-resistant double-coatedsteel material in accordance with claim 1 in which the tin-predominantouter coating consists of a tin-based alloy.